Automatic transmissions comprise a plurality of gear trains. Their speed ranges are selected among a plurality of power transmission paths which are composed of these gear trains by actuating such frictionally engaging elements as clutches and brakes through the supply of hydraulic pressure. When a speed range shift is made, the power transmission paths switch to change the transmission gear ratio. If a shift is carried out suddenly, there arises a problem of shift shock. Therefore, various ideas have been contrived to adjust engagement of the frictionally engaging elements in order to carry out speed-range shifts smoothly without any shock.
This shift shock problem is especially noticeable if the shift lever is operated from the neutral range to the forward range (which is referred to as "in-gear shift"). The reason is that there is a little input torque but a large ratio of transmission torque change to engaging capacity change in the frictionally engaging element involved in the in-gear control, through which the transmission shifts from the neutral range having no load to the forward (or reverse) range: Therefore, this frictionally engaging element requires delicate engagement-actuation control.
Various ideas have been proposed for the in-gear control. For example, Japanese Laid-Open Patent Publication No. 6(1994)-109130 proposes a control apparatus which executes a first shift-control stage for executing a fast quick-fill by actuating the solenoid valve at a large duty cycle when a shift command to shift from the neutral range to the driving range is generated. Then, the control apparatus executes a second shift-control stage for reducing the pressure supplied to the clutch by actuating the solenoid valve at a predetermined duty-cycle reduction rate. Before completely engaging the clutch, it executes a third shift-control stage for minimizing the pressure by actuating the solenoid valve at a duty cycle determined on the basis of the rotational speed of the engine.
There is an in-gear control in which a higher speed range is temporarily set before establishing a first speed range (lowest speed range) when a shift is made from the neutral range to the forward range. This control, which is referred to as "squat control"because the car tends to move downwardly or "squat" upon a sudden torque change, smoothes out the change of output torque during the shift to the forward range, thereby alleviating the shift shock problem. Specifically, Japanese Laid-Open Patent Publication No. H3(1991)-6390 discloses such shift control method. When the shift lever is operated from the neutral range to the driving range while the vehicle speed is approximately zero with the accelerator pedal released and the parking brake applied, the lowest speed range (first speed range) is established after a higher speed range has been set temporarily.
As the above mentioned in-gear control is intended only to shift the transmission from the neutral range to the driving range smoothly, for example, if the shift lever is operated to the driving range soon after a shift from the driving range to the neutral range, the in-gear control of the prior art has experienced a problem of shift shock or shift delay.
When a shift is made from the driving range to the neutral range, the hydraulic pressure in the frictionally engaging element for establishing the driving range is released. However, if a shift is made to the driving range immediately after the shift from the driving range to the neutral range, then the frictionally engaging element for establishing the driving range is resupplied with hydraulic oil while some oil from the previous actuation is still remaining there. This condition makes the frictionally engaging element engage suddenly, resulting in a problem of shift shock.
For example, if the shift lever is operated in such a way with the control apparatus disclosed in Japanese Laid-Open Patent Publication No. H6(1994)-109130, then the first shift-control stage is executed while some oil is still remaining in the frictionally engaging element for establishing the driving range. Because the solenoid valve is actuated at a large duty cycle for a fast quick-fill, the actuation pressure of the frictionally engaging element reaches a high pressure quickly, thus causing a shift shock. In addition, as the shift control executed by this control apparatus requires some time to repeat the first through third shift-control stages for each shift, it tends to present a problem of shift delay as well.
Especially, if a shift is made to the driving range immediately after the shift from the first speed range to the neutral range with the above mentioned squat in-gear control, then a shift delay or a shift shock is all the more likely. The reason is that the squat in-gear control establishes the first speed range through temporary establishment of a higher speed range while it would be better for the control to keep the first speed range as is.
Automatic transmissions include not only the above described type which comprises gear trains but also a type which comprises a continuously variable speed mechanism. Since continuously-variable-type automatic transmissions also allow shifts to a forward range, neutral range, and reverse range in response to operation of the shift lever, the above mentioned same problem occurs when in-gear control is executed.